This invention relates to electromechanical vibrators, and more particularly, to audio-frequency electromechanical vibrators adapted for a body-felt vibration reproduction in sound reproducing systems.
Sound reproducing systems that impart a body felt vibration are well known in the prior art. For example, U.S. Pat. No. 4,064,376 discloses an electromechanical vibrator which reproduces from an electric signal not only sound that is heard but also mechanical vibrations, preferably undertones lower than 150 Hz, that are directly transmitted to a body. The electromechanical vibrator for reproducing the mechanical vibration is fitted on a bed or a chair, and an audio signal that is fed to the sound reproducing speakers is also applied to the vibrator, usually after passing through a filter for removing frequency components higher than 150 Hz. A person on the chair or bed feels vibration while listening to the music.
An electro-dynamic transducer is used for the vibrators in such sound reproducing systems. A known vibrator has an arrangement similar to the electro-dynamic speaker shown in U.S. Pat. Nos. 4,064,376 and 4,354,067. FIG. 1 shows a vertical sectional view of a known audio-frequency electromechanical vibrator. The vibrator 1 includes a magnetic circuit comprising an inversed T-shaped magnetic yoke 2 having a center pillar 2a and a bottom plate 2b, a ring-shaped permanent magnet 3 disposed on the bottom plate 2b, and an annular top yoke plate 4 attached to permanent magnet 3. An annular small space or magnetic gap 5 is formed between a top portion of center pillar 2a and an inner end of annular plate 4. A drive coil 6 is loosely fitted or disposed in magnetic gap 5. The magnetic circuit structure is elastically supported by a case 7 through a spring plate 8, and drive coil 6 is supported by a case cover 9 through a coil bobbin 10.
Since drive coil 6 is disposed in a static magnetic field generated in the magnetic gap 5, drive coil 6 and the magnetic circuit structure (items 2, 3 and 4) are relatively moved when an electric A.C. current is applied to drive coil 6. Thus, the vibrator 1 vibrates in response to an electric audio signal applied to drive coil 6.
Employing this construction, the transmit efficiency on vibrator 1 is determined by the magnetic flux density of magnetic gap 5, and the number of turns and impedance of drive coil 6. Therefore, in order to improve the efficiency of vibrator 1, the size of the magnetic gap 5 should be reduced, and the number of turns and diameter of the wire of drive coil 6 should both be increased. However, drive coil 6 must be placed in magnetic gap 5 without any contact with other moving parts. Thus, drive coil 6 is formed of a small coil of thin wire with high density, and requires a high number of turns to improve the vibrator's efficiency. As a result of the above requirements for improving the efficiency of vibrator 1, high accuracy is required when producing the coil and manufacturing the vibrator which will finally increase the cost of the vibrator.
Also, as is clearly shown in FIG. 1, center pillar 2a of magnetic yoke 2 is disposed in drive coil 6 to generate the vibration. Thus, the thickness of vibrator 1 cannot effectively be reduced for easily fitting the vibrator into a sheet or chair.